Using quantum Monte Carlo simulations and field-theory arguments, we study the fully frustrated transversefield Ising model on the square lattice for the purpose of quantitatively relating two different order parameters to each other. We consider a "primary" spin order parameter and a "secondary" dimer order parameter, which both lead to the same phase diagram but detect Z(8) and Z(4) symmetry breaking, respectively. While at T > 0 their scaling exponents are simply related to each other, as explained by a mapping to a height model, we show that at T = 0 they correspond to different charge sectors of the O(2) model in 2 + 1 dimensions with nontrivial exponents that are not simply related to each other. Our insights are transferrable to a broad class of Ising models whose low-energy physics involves dimer degrees of freedom, and also serve as a guide to treating primary and secondary order parameters more generally.